For decades the polling firm IPSOS has been asking Brits to nominate the most important issue facing their country. In 2010, just after David Cameron became prime minister, how many people do you think named the EU as their top concern?

It was 1 per cent, the lowest figure IPSOS ever recorded.

Even for the next five years, that number never rose above 15 per cent. Then in late 2015 it climbed sharply in the lead-up to Cameron's Brexit referendum.

What happened? How did an issue that was so marginal in 2010 that it barely registered in an opinion poll come to create Britain's biggest foreign-policy upheaval since the Suez crisis?

Clearly, the British public was not clamouring for a reckoning with Europe. That sentiment was confined to a small minority of voters and, crucially, to the Eurosceptic wing of David Cameron's Conservative Party. Cameron bent to pressure from that faction, and to worries over the growing popularity of Nigel Farage's UK Independence Party, and promised an in/out referendum.

Because Cameron had insufficient authority within his own party to suppress its Eurosceptic wing, he had to outsource an internal party dispute to the public. A small minority of Tory MPs faced down a vulnerable leader, and the result is that Britain's place in the world will now undergo a seismic shift.

Australian similarities

The broad scenario is not entirely unfamiliar to Australians. From 2015 to 2017 the Liberal Party too faced an internal dispute which it simply could not resolve within the party room, thanks in part to two successive leaders (Abbott and Turnbull) who suffered unmanageable discontent from energetic factions. The issue was same-sex marriage, and the Liberals eventually did what David Cameron did, which was to call on the public to decide what they could not.

Unlike Brexit, same-sex marriage won't realign Australian foreign policy. But the political conditions that created the postal plebiscite have not changed. So keep your eye on other issues that tend to be uncontroversial among the general public but which energise small groups of politicians and activists.

Recently, senior Liberals such as Peter Dutton and Tony Abbott have floated the idea of a temporary reduction in immigration numbers to give Australia a chance to "catch up" with growing demands on housing and infrastructure. But Abbott in particular has stressed that this would be a short-term measure, and he has otherwise put himself firmly in the mainstream: "Immigration is at the heart of who we are. The fact that so many millions have come here to build a better life, originally from the British Isles but then from the four corners of the earth, lends a heroic dimension to our national story."

What if a group of Liberal MPs took a more radical approach? Or what if a faction of the Labor Party decided that, to paraphrase former NSW Labor Premier Bob Carr, Australia is full? Brexit shows that in an era in which leaders command less authority and parties are becoming weaker, a pet issue among a small minority of MPs can quickly go mainstream.

If an "Australia is full" movement ever got that kind of momentum, voters could find themselves facing a decision just as momentous as the Brexit referendum. An election in which one side called for permanent large-scale cuts or even a population cap would effectively be a referendum on Australia's multicultural identity and its future in the region. If a policy of that kind was ever implemented, Australia would symbolically turn its back on the world and face a rising Asia as a nation in decline.

There is good reason to think that the great mass of Australians would reject the "Australia is full" argument if they ever had to vote on it. Australians can be proud of the way they have welcomed waves of immigrants for generations to create one of the most successful and least racially divided countries on earth.

Then again, Brexit did not happen due to a groundswell of public sentiment either. Just six years before the referendum, 99 per cent of the British public could not have cared a fig about the EU, but politicians forced them to choose.

David Cameron and the entire British political establishment were confident Brits would make the right choice, yet it emerged that the voters' commitment to the nation's direction as an ever-closer part of Europe was far lower than anticipated. For the political class, this direction was almost sacrosanct; their shock at the referendum result indicated that they simply had not imagined an alternative. But it turned out that just enough Brits did imagine a future for their country radically different to the one the political class had arranged for them.

Australia has seen decades of sustained bipartisanship on immigration policy, but we may find that beneath it is a sea of voter indifference. Brexit is a warning of what happens when that indifference is disturbed.